Building Britain's first high speed line

01 May 1999

Section 1 only impinges on the urban fabric at Ashford, but Section 2 penetrates to the heart of London where driving 36 km of single-track bored tunnel through some difficult ground beneath existing railways and other structures presents a major engineering challenge

THE DECISION to split construction of the Channel Tunnel Rail Link into two sections stems as much from an assessment of engineering risk as from London & Continental Railways' failure to finance the project as a single entity, though the two are clearly linked.

Section 1 is quite straightforward, with the exception of some complexity in Ashford where grade-separated connections have to be made with Railtrack's existing lines in order to divert trains off CTRL and into platforms 3 and 4, which are used exclusively by Eurostar today. There is only one major tunnel to be bored through the North Downs, and one major bridge crossing the River Medway at high level. Target cost for the 74 km of double track is £1·7bn.

Section 2 is a very different proposition, with a target cost of £2·5bn for only 39 km. There are two intermediate stations to build at Ebbsfleet and Stratford, a tunnel under the Thames estuary, some highly complex works in and around the historic St Pancras terminus in London, and above all, the boring of parallel single-track tunnels totalling 36 km in all between St Pancras and the outer fringe of suburbia east of Barking.

Design parameters

CTRL will be a double-track railway electrified at 25 kV 50Hz. Over most of its length, the design allows for trains with a maximum axleload of 17 tonnes running at 300 km/h using transmission-based signalling. Speed will be reduced to 230 km/h between Ebbsfleet and St Pancras, primarily for aerodynamic reasons in the tunnels.

The track will consist of UIC 60 kg/m rail on concrete sleepers in granite ballast for the bulk of Section 1, but concrete slab track will be used in the tunnels and on some structures in Section 2. The ruling gradient has been set at 2·5%, which will occur over short distances, notably the 1 km long connection between the Dollands Moor freight yard near Folkestone and the CTRL Up track which crosses over the existing Saltwood Tunnel.

Likewise, minimum curve radius on 300 km/h sections has been set at 4000m. This is influenced by the fact that CTRL will be used by commuter trains limited to a maximum speed of 200 km/h, and possibly by fast freight services at 140 km/h. The maximum cant of 180mm therefore represents a compromise.

On Section 1 adjacent tracks are 4500mm between centres, and the standard height of the contact wire is 5080mm. CTRL will be built to the UIC-GC loading gauge, which will accommodate complete lorries piggybacked on flat wagons. Freight, if it materialises, is expected to leave CTRL at Ripple Lane where grade-separated connections are being provided.

Cheriton to Fawkham Junction

Section 1 joins up with existing tracks inside the boundaries of Eurotunnel's UK Terminal at Cheriton, 109 km from St Pancras, and therefore requires a new bridge over the M20 motorway. Up and Down CTRL tracks are widely separated as they pass south and north of Continental Junction and Dollands Moor yard, the Up track passing in shallow cutting over the 800m Saltwood Tunnel opened in 1842, and currently used by all international trains.

The two tracks converge and are joined by the freight link from Dollands Moor before descending to run along the north side of the existing line to Ashford. The first of several short cut-and-cover tunnels is located alongside the existing Sandling Tunnel, thus honouring an agreement reached 160 years ago by the South Eastern Railway with a local landowner. Another, embracing both railways, will reduce severance in the village of Mersham.

Entering Ashford, CTRL rises on viaduct to pass over the Canterbury line, and as it does so, throws off grade-separated connections into the station which is presently electrified at 750V DC third-rail. Tracks through the station used by Eurostar will be wired at 25 kV to avoid the complications of a double traction changeover.

Westinghouse has been awarded a £6m contract to expand the capacity of the signalling installed at Ashford in the early 1990s to control much of the route normally used by Eurostar. This is specifically to cope with changes during the work in this area, but it is likely that the Ashford signalling centre will also control the CTRL now that has been taken over by Railtrack.

CTRL itself passes through a gap left between the domestic terminal building and the platforms when the station was totally rebuilt in the mid-1990s - though nobody was prepared to fund the corresponding opening in the adjacent Beaver Road bridge when rebuilding took place about five years ago. It then dives into a 1·7 km cut-and-cover tunnel under the Maidstone line and an industrial estate, picking up connections to be used by trains calling at Ashford within this structure.

The line swings north to join the M20 at Tutt Hill, where the principal rail-connected construction site, known as Beechbrook Farm, is to be established. Tracklaying is expected to commence at this point, heading in both directions.

From Beechbrook to Detling the line stays close alongside the M20, except where curvature of the motorway is too tight. Four villages enjoy the protection of cut-and-cover tunnels a few hundred metres long, despite having a six-lane road close alongside. Although CTRL was already intended to follow this route when the M20 was built in the early 1990s, the opportunity to construct numerous bridges with openings for the railway alongside was missed.

In selecting the best alignment for the CTRL, every effort was made to place it alongside existing railways or roads to minimise severance and environmental impact. A short exception is the 9 km west of Detling where the line switches across from the M20 to the M2, but 3·2 km of this distance is in tunnel under the North Downs ridge.

Construction of the North Downs tunnel as a double-track horseshoe section 13m wide and 10m high will be undertaken by roadheaders and backactors to load the spoil. A concrete lining will be sprayed on, with reinforced concrete segments installed later to form a permanent lining.

The drive will be uphill from both portals to a mid-tunnel summit, passing through Lower Chalk and Middle Chalk, which is above the water table. Maximum cover is around 90m.

The line descends into the Medway valley alongside the M2, which is currently being widened on the south side. The tidal Medway is already crossed by the M2 on an impressive bridge, and this is being duplicated for the road widening.

To minimise visual intrusion, the rail bridge will have the same profile as the road bridges alongside. This means that the rails will be 30m above high water level in the river and 37m above the riverbed. The central span is 152m and the total length of the structure is 1·26 km. The height of the Medway bridges is, in fact, set by the need for the M2 and CTRL to pass at right angles over the higher of two parallel railways built by rival companies along the west bank.

From the Medway, the line follows the south side of the M2 and A2 roads for a further 11 km to the point where Section 1 of the CTRL proper ends at a junction to be known as Southfleet. The formation of a former branch line to Gravesend West is used to bring trains onto the existing main line between London and Chatham at Fawkham Junction. A stretch of 25 kV catenary will be erected along this line to facilitate the changeover by Eurostars to third rail power for the rest of the journey to Waterloo.

Railtrack will spend up to £65m to upgrade the route between Fawkham Junction and Waterloo. This includes provision of reversible signalling between Fawkham Junction and Swanley, and a new high speed crossover at Swanley. Clearance for Class 92 locomotives via Redhill, thereby releasing paths in the Swanley area, is also being considered.

Southfleet to St Pancras

The very first work on the CTRL began at Pepper Hill, just beyond Southfleet Junction, on June 7 1997. It involved the construction of tunnels under the A2 to permit high voltage cables into a grid substation to be diverted, clearing the way for a short tunnel for CTRL to be driven under the A2 at this point.

This brings the line into the proposed Ebbsfleet station, which is located to capture motorists using the M25 motorway around London. There will be a pedestrian link with Northfleet station on the North Kent line, and this is also the point at which connections will permit commuter trains from Gravesend, Chatham and Margate to join CTRL to reach St Pancras. Commuter trains from Ramsgate, Canterbury, Dover and Folkestone will join CTRL at Ashford.

The line immediately plunges into the two 2·5 km single-track bored tunnels under the tidal Thames, which is over 1 km wide at this point, emerging in Purfleet.

Threading on an elevated structure between the supports of an approach viaduct carrying the M25 on to the Queen Elizabeth II bridge over the Thames, the CTRL picks up the alignment of the Tilbury loop of the London, Tilbury & Southend line east of Purfleet and follows on the south side until the portal of the London tunnel is reached at Ripple Lane. Freight connections diverge here.

When the go-ahead for Section 2 is received, the first contract signed will be for the TBMs that will undertake the eight drives required to create the London tunnel. These will have to be closed-face machines capable of boring through water-bearing sands and gravels. The terrain is relatively flat and the depth of the tunnels is dictated by geology. The tunnels will be lined with concrete segments, and will be driven simultaneously west from Ripple Lane, east from St Pancras portal, and in both directions from Stratford.

The distance between the two portals is 19 km, but almost in the middle there will be an open box 1070 m long, 50 m wide and 13 m deep for Stratford station. This will contain two platforms for international trains and two for domestic trains serving Kent, as well as through tracks for non-stop trains. As at Ashford, domestic and international passengers will be segregated to comply with Britain's strict frontier control regime.

This box is large enough to contain future connections facing towards St Pancras for a rolling stock depot to be built at Temple Mills nearby, which may be required if UIC gauge stock uses CTRL. Meanwhile, Eurostars terminating at St Pancras will reach their existing North Pole depot via the North London line.

For safety reasons, the London Tunnel will consist of two single-track bores of 7·15 m internal diameter connected by cross-passages at 750 m intervals. There is a safety requirement for emergency access shafts at regular intervals. Emergency fans for smoke control and other plant in the tunnels will be provided, but the piston effect of the trains will ventilate them naturally in the normal way.

Trains arriving from France will emerge from the tunnel into a complex series of connections to be constructed north of the St Pancras and King's Cross termini. They will also be able to carry straight on past St Pancras to Birmingham or Manchester - although the present operators of Eurostar see no commercial case for this at present - or continue into the rebuilt and expanded station at St Pancras.

An integral part of the work necessary in the St Pancras area is construction of a station box and tunnel connections to the East Coast Main Line for the cross-London Thameslink 2000 project. The box would be partially underneath the structure supporting extended platforms required by Eurostar, and London & Continental Railways has an obligation to provide it as part of CTRL Section 2. The government appears content to wait until there is a decision on Section 2 before funding the Thameslink box through LCR.

CAPTION: The North Downs tunnel portal at Blue Bell Hill south of Chatham is taking shape rapidly alongside the M2 motorway

CAPTION: The first phase of the Channel Tunnel Rail Link mostly runs alongside the M20 and M2 motorways before taking the alignment of the abandoned Gravesend West branch to Fawkham Jnc, where trains will join the existing tracks to reach Waterloo